Electronic display devices have another name of "man-machine interface", which have an important role of "interface" to transmit various visual information to "man" from "machine" while connecting "man" and "machine" via "interface".
Known are two types of such electronic display devices, emissive one and non-emissive one. Emissive display devices include, for example, CRT (cathode ray tube), PDP (plasma display panel), ELD (electroluminescent display), VFD (vacuum fluorescent display), and LED (light emitting diode). None-emissive display devices include, for example, LCD (liquid crystal display), ECD (electrochemical display), EPID (electrophoretic image display), SPD (suspended particle display), TBD (twisting ball display), and PLZT (transparent ferroelectric PLZT [(Pb,La)(Zr,Ti)O.sub.3 ] ceramics display).
For multicolor imaging in electronic display devices, for example, known are a method of disposing multicolor emitting zones (for example, for three primary colors of red, blue and green) with being laterally spaced to separately emit the intended color, and a method of disposing a plurality of different color-changing layers (for example, color filters or phosphors) in which those color-changing layers receive monochromatic light and separate or change its color to emit different colors.
ELD is characterized by its high visibility as being self-luminescent, and by its high impact resistance as being completely solid. At present, various types of ELD are being developed, comprising inorganic or organic compounds in their light-emitting layers. Of those, organic EL devices (organic ELD) comprising organic compounds as sandwiched between two electrodes are greatly expected to be in displays capable of efficiently emitting high-luminance light in various color regions, since plenty of different organic compounds are employable therein.
Turning to the current multicolor imaging methods using those organic EL devices, the method using ELD that comprises different color emission zones as laterally spaced to produce different colors is problematic in that it requires different light-emitting materials for different colors and that the materials, as being organic compounds, are poorly resistant to the working (e.g., photolithography) of laterally spacing them on substrates. Therefore, the method using ELD that comprises a plurality of different color-changing layers capable of separating or changing the color of monochromatic light into different colors is preferred, as being simple, since light-emitting layers for monochromatic light only may be provided in ELD.
In ELD having color filters as the color-changing layers, light loss is great because of the function of the color filters through which the color of light is separated or cut. For example, where the color of white light emitted is separated into three primary colors (red, green, blue) through color filters, the white luminance is reduced to at most 1/3.
On the other hand, in ELD having color-changing layers of phosphors, the layers have the function of absorbing light to change it into longer wavelength fluorescence with smaller energy. For example, if phosphors having a degree of light absorption of 80% emit fluorescence at an yield of 80%, they can change light into longer wavelength light at an yield of 64%. In fact, known are phosphors of that type. Accordingly, color-changing layers of phosphors are preferred in ELD, as being able to efficiently utilize light.
Known are some multicolor structures to be in organic EL devices, in which monochromatic light is changed into different colors through a plurality of different color-changing layers of phosphors, for example, as in Japanese Patent Applications (JP-A) Laid-Open Nos. 3-152897 and 5-255860. In JP-A 5-255860, disclosed is an image display device as in FIG. 11, in which fluorescent media are so disposed that they can receive emitted light from the organic EL medium.
However, as having no light-shielding layers, the disclosed image display device is defective in that light as isotropically emitted by the organic EL medium passes through the insulative planarizing layer (light-transmissive medium) to penetrate into not only the intended fluorescent medium but also the adjacent fluorescent medium thereby causing color mixing.
The disclosed image display device has a laminate structure comprising an organic EL medium as superposed over fluorescent media, in which the organic EL medium of a thin layer is sandwiched between two electrodes. In this, therefore, the influence of the surface roughness of the underlying fluorescent media on the organic EL medium is great. Specifically, the rough surfaces of the underlying fluorescent media will cause leak between the two electrodes and will even break the connection therebetween, thereby having some negative influences on the driving capability of the device and, after all, lowering the production yield of the device.
In JP-A 5-258860, disposed is an insulative planarizing layer between the fluorescent media and the organic EL medium, thereby compensating the surface roughness of the fluorescent media. In this, however, disclosed is no technique of unifying the different fluorescent media to thereby planarize those media all at a time. In JP-A 5-258860, the thickness of the fluorescent medium is defined to be smaller than 10 .mu.m. However, the fluorescent medium having a thickness of smaller than 10 .mu.m is unsatisfactory for emitting fluorescence.